Line a bowl with a clean, sturdy towel. Grate cucumbers into the bowl. Gather towel around cucumber and squeeze as much liquid from the cucumbers as possible. Transfer cucumber to a colander and sprinkle 3/4 teaspoon salt over cucumber. Stir to evenly coat cucumber and allow to drain, 10 to 30 minutes.
Mix cucumber, olive oil, vinegar, red onion, garlic, dill, 1/4 teaspoon salt, and ground black pepper together in a bowl.